HOW RARE IS RARE?
According to the National Organization for Rare Disorders, “rare disease” in the U.S. means affecting fewer than 200,000 people. These conditions number about 6,800, collectively affecting nearly 30 million Americans or 1 in 10 people. Many are single-gene diseases. That means that the chance of more than one family member being affected is quite high (see Mendel's first law). Unlike those, most (>90%) cases of ALS and breast cancer aren’t inherited as single-gene traits, but are sporadic. Mutations happen during a person’s lifetime in somatic cells, perhaps due to an environmental trigger. A family with one member who has ALS wouldn't have as great a chance as it affecting another as a family with Huntington disease, for example.
With so many causes of rare diseases, comparing statistics is an apples-and-oranges exercise. But I collected a few anyway, for prevalence (the percentage of a population with a particular disease at a given time). (more…)